UFS2: make di_extsize unsigned.

di_extsize is the EA size and as such it should be unsigned.
Adjust related types for consistency.

Reviewed by:	mckusick (previous version)
MFC after:	3 weeks
Add #ifdef wrapper around definitions so they aren't included multiple
times.
Break out the debug code into a new include file in preparation for
some more iwn work.
Return 0 if:

  1. "-u N" specified, no -f, and mdN found,
  2. no -u, "-f /pathname" specified, and mdN associated with /pathname found,
  3. "-u N" specified, "-f /pathname" specified, and both of them found,
  4. "-l" specified and no -f,
  5. "-l" specified, "-f /pathname" specified, and /pathname found.

otherwise return -1.

Spotted by:	Julian H. Stacey